This invention relates generally to inverter-driven induction motor systems and, more particularly, to a system and method for significantly reducing the undesirable radio frequency energy emitted from the inverter drive system.
Adjustable frequency drives for controlling the speed of an induction motor are well known. Such drives often comprise a static inverter for rectifying a three-phase alternating current (ac) input signal into a direct current (dc) intermediate signal which is then chopped into a variable voltage, variable frequency ac output signal used to drive the motor. These inverters may be of several types, one common type including power switching elements such as insulated gate bipolar transistors (IGBTs) gated by pulse width modulation (PWM) firing signals.
However, such IGBT-based inverters can emit excessive radio frequency (rf) energy often referred to as electromagnetic interference (EMI). High IGBT firing speeds can induce current changes from 1000-3000 amps/.mu.sec and voltage changes up to 5000-6000 V/.mu.sec, these changes appearing as edges on the PWM waveform. The leads from the inverter drive to the motor act as an antenna, transmitting this high frequency energy into space as interference. All power lines in the surrounding vicinity, including the power line to the inverter itself, then act as receiving antennas, picking up the emitted energy. This undesirable excess energy can interfere with certain types of equipment, particularly radios, computers and other such electronic devices. With increasingly stringent limits being put on the amounts of such interfering emissions, such as those contained in the proposed standards of the CISPR (International Special Committee on Radio Interference), inverter manufacturers have had to address electromagnetic capability and find further ways to ameliorate the effects caused by high dv/dt switching common with modern IGBTs.
The system and method of the present invention address this concern by providing a low cost solution to excess radiation conduction and emission from IGBT-based inverter drives. To accomplish this, a four winding common mode inductance is added on the output side of the inverter drive. A ground wire choke as well as capacitive and inductive filtering elements are also added to the inverter input circuitry to reduce EMI. These modifications provide an inverter drive system well within compliance with the newly emerging standards. In addition, they are relatively low in cost and are readily implemented.
These and other features and advantages of the present invention will become apparent upon review of the following description, taken in conjunction with the accompanying drawings.